High-speed flywheels (e.g. those running at speeds of 20,000 rpm or more) for use as energy storage devices in hybrid vehicles are well known. One of the challenges presented by this technology is Noise, Vibration and Harshness (NVH). The flywheel is often at high speed (and therefore produces the most amount of noise) when the vehicle is at low speed or even stationary with the engine turned off.
Furthermore, although it is possible to separately design a flywheel and a flywheel housing each having no resonant modes within the flywheel operating speed range, when the two components are joined together, the interaction of the flywheel and the housing may produce a multi-body resonance which does occur in within the flywheel operating speed range.
An aim of the present invention is to reduce the effect of NVH on a hybrid system, and which ensures that the natural modes of the flywheel do not interfere with the natural modes of the housing and vice versa, thereby allowing full speed operation of the flywheel without producing damaging resonance.